It is known that in-line optical fiber gratings can be formed by applying two interfering beams of ultraviolet radiation to a glass waveguide structure. See, for example, U.S. Pat. Nos. 4,807,950 and 4,725,110 which are incorporated herein by reference. Typically these gratings are formed in Ge-doped optical fiber.
The present applicants have discovered and disclosed in the aforementioned parent applications that the index-changing effect of actinic (UV) radiation can be enhanced by treating the glass with hydrogen or deuterium.
The present application is predicated upon the further discovery that in hydrogen or deuterium treated glass (hereinafter generically referred to as hydrogen-treated glass), the index of refraction can be increased not only by actinic radiation, but also by the application of heat.